bun: test coverage is thin

bun test reports 50 tests across 5 files. Cloudflare has 929, hono 241.
No tests at all for plugin.ts, client.ts, integrations/fetch.ts.
dev-packages/bun-integration-tests has 5 suites: basic, fetch, hono-sdk, orchestrion-mysql, orchestrion-postgres. Deno has 24.
Bun tests run under bun test, not vitest, so yarn test at the repo root does not run them. Easy to forget locally.

Work item. Pick the three highest-value additions: plugin.ts (the only instrumentation path that actually works on Bun today), streaming responses once B3 lands, and one more real orchestrion integration suite that goes through Bun.build (redis or express).

**Prior art **(related history only). #18635 (open) "Expand canary test coverage across all framework SDKs" is the nearest umbrella. No issue covers the Bun unit or integration test gap specifically.